When measuring contents levels a difference is made between devices measuring the contents level continuously, which can give a measured value at all times of the current contents level in the container, and limit sensors, which only display whether the contents level in the container is above or below a predefined level to be monitored by the limit sensor. Such limit sensors are especially used for monitoring a maximum or minimum contents level in order to prevent overfilling or underfilling of the container. Different types of limit sensors are known, for example capacitive or resistive limit sensors, which respond to the electrical properties of the material in the container, limit sensors with oscillating elements, such as membranes or vibrating rods, whose vibrations are damped by the material in the container or whose self-resonant frequency is modified by the material in the container, etc. The selection of the limit sensor used depends on the properties of the material in the container and on the application conditions. In each case, a limit sensor gives an electrical signal which has a first signal value when the contents level is below the level to be monitored, and which has a different second signal value when the contents level is above the level to be monitored. The signal changes its signal value at exactly the moment the contents level passes the level to be monitored through addition of material to or removal of material from the container. With several types of limit sensors the signal value depends on whether the limit sensor is in contact with the material in the container or not; with such limit sensors the predefined level of contents monitored by the limit sensor corresponds to the level of installation of the limit sensor.